Rivers and catchments of Cape Town

4.1.3 Diep river catchment

The Diep River has its source outside of Cape Town. The river rises in the Perdeberg and Riebeeck-Kasteel Mountains to the east and north of Malmesbury and flows in a south-easterly direction for approximately 65 kilometres. This vlei is located on the Table Bay coastline between Milnerton and Table View. The Diep River is linked to the sea via the Milnerton Lagoon. Various tributaries join the Diep River on its way to the sea.

Zoarvlei wetland is the connection piece of the former Salt River that was cut off from the Diep River in the north and the Salt River canal in the south. Originally wetlands framed and buffered it, but due to development of the adjacent areas it has diminished over the years. “Development in the surrounding area destroyed the ecologically important links with both these systems. It exists today as a seasonal,freshwater coastal lake fed by groundwater intrusion and stormwater runoff, with a small outlet to the sea via a culvert leading into the Milnerton Lagoon”(Cate Brown and Rembu Magoba, 2009). As a wetland it is protected in its current state. It is the most southern entity of the Cape Wet Coast Biosphere Reserve. “Despite its poor ecological condition, Zoarvlei performs a vital role in the successful migration of birds. Approximately 125 species of birds have been recorded in the wetlands, many of them rare. In recent years”(Cate Brown and Rembu Magoba, 2009).

Before entering the sea, the Diep River flows through the Rietvlei wetland and the Milnerton lagoon, which togeth- er cover an area of approximately 900 hectares. These two features together have generally been considered to comprise the “estuary”. Water quality problems are the major concern in the wetland. “The treated waste water from the Potsdam Waste Water Treatment Plant (WWTP) is discharged to a channel along the eastern boundary of Rietvlei which conveys the effluent stream to the head of the lagoon at the Otto du Plessis road bridge”(PEAK, 2008). The estuary type is a temporary open estuary with a bar that breaches. “Over a period of around 20 years from the early 1970’s until the construction in 1991/92 of the channel associated with the sewage works, the mouth closed on a regular basis albeit for varying periods. It was then either breached by floods, or was artificially opened by the town engineers once the water level in the lagoon reached between 1.9 and 2.0 m above MSL. Since the construction of Woodbridge Island and the channel, the mouth has again remained open” (PEAK, 2008). This mouth was called the second mouth in the historical maps since 1860. “The periodic closure of the mouth was probably due to both reduced water flows, and siltation, resulting in reduced tidal inflows that they were no longer strong enough to keep the mouth open” (PEAK, 2008). This strenghtens the opinion that the mouth was rather constructed.

Initially it was envisaged to use the Leitbild method of the EU Water Directive. The Diep river estuary could have been a reference status for the Salt River rehabilitation.

Due to the Covid-19 pandemic, it was not possible to obtain the necessary information or do site visits.

The Diep River estuary has many of similarities with the Salt River due to them belonging once to the same system. They are situated geographically at the same coastline a couple of kilometres apart. The Diep River was before the separation the upstream reach of the Salt River. Another similarity is that it receives effluent from a WWTW as well. The catchment area is much bigger and has a bigger proportion of agricultural land, whereas the industrial land is not part of the estuary itself as in the Salt River.

4.1.4 Climate

Cape town has a winter rainfall and dry summer climate, that is often compared to the mediterranean climate. The average annual temperature is a mild 17°C. June has the highest rainfall with cool temperatures, low wind speed and a high in humidity.

In summer blows a strong South easterly wind over the Cape flats with the highest temperatures in January/February and a low humidity, which brings a high evaporation rate.

In winter the main wind direction is North westerly, which can bring storm surges along the Atlantic coast. The precipitation rate within the metropolitan boundary varies between 300-1000mm and is influenced by the outcrops of the Tygerberg hills and the Table Mountain group.

4.1.5 Geology and Soils

The study area is situated on two geological groups the N-Qs - Quartzose sand, pelletal phosphorite, that is characterized as gravel, sandy silt, grey-black carbonaceous kaolinitic clay, peat, shelly limestone and sandstone, shelly sand and (aeolian) calcarenite, coquinite, light grey to reddish sandy soil, loamy sand the second group is the CRtPhyllite, meta greywacke, quartzite, minor volcanic rocks.

The geology of the area is characterised by Sandveld Group sands, Quaternary aeolian sands characteristic of the Cape Flats area. In the Cape Flats District, the Sandveld Group is mainly represented by the Springfontyn Formation, which was developed through the deposition of windblown sand (an aeolian deposit) and consists of reddish to grey, unconsolidated quartzose aeolian sand.

4.1.6 Ecosystem protection levels

The South African National Biodiversity Institute (SANBI) developed as part of the National Biodiversity Assessment (NBA) in 2018 national data on the threat and protection status of Marine and Freshwater systems. The NBA is the primary tool for monitoring and reporting on the state of biodiversity in South Africa. The marine and aquatic inland system are illustrated together in two diagrams, as the freshwater system impacts on the receiving marine life and vice versa. The fact, that the two systems influence each other is reason enough to protect and enhance them. While the Table Bay Marine life is moderately protected the two estuaries are only poorly protected, diagram 55. Apart from the sewage outfalls, the pollution from the harbour the Salt and Diep River with their inflowing water have a major impact on the health of the Table Bay. This is one of the reasons to help the estuaries to achieve a good ecological status. "River ecosystem types are Poorly Protected with only 13% considered Well Protected and 42% Not Protected" (SANBI,2018). The Salt River Estuary is poorly protected, while the Salt River canal is not protected.

4.1.7 Ecological state categories

"The Cool Temperate bioregion is characterised by estuaries in a heavily modified or worse state" (SANBI,2018). "Nearly 30%of salt marsh habitat has been lost as a result of poor land-use practices, flow reduction and related mouth closure, direct harvesting and overgrazing. Several estuarine-dependent fish species are threatened by overfishing (especially gillnetting),declining water quality, and reduced flows with their concomitant influence on recruitment and marine connectivity. An estuary was assigned a condition category ranging from natural (A) to critically modified (F), which relate to decreasing levels of ecosystem function" (SANBI,2018). Both Salt River canal and estuary are critically modified.

4.1.8 Ecosystem threat status

"The estuarine realm is the most threatened of all realms in South Africa, both for the number of ecosystem types (86% threatened) and for area (99% threatened). Estuaries are under-protected in South Africa with only 18% of ecosystem types and 1% of estuarine area Well Protected" (SANBI,2018). "River ecosystem condition declined by 11% between 1999 and 2011. Of the 222 river ecosystem types assessed, 64% were found to be threatened (43% Critically Endangered, 19% Endangered and 2% Vulnerable). Ecosystems with <20% of their spatial extent in a natural/near-natural ecological condition, were considered Critically Endangered. Thresholds of 35% and 60% were used for Endangered (EN) and Vulnerable (VU) categories, whereas ecosystems with >60% in a natural or near-natural ecological condition were considered of Least Concern (LC)" (SANBI,2018). The threat status of Salt River estuary and canal is critically endangered. The Table Bay marine life is under threat to become endangered and the Diep River and Salt River estuaries are critically threatened, see diagram 57.